Energy emitted in the form of waves or particles is referred to as radiation. A common type of radiation is electromagnetic radiation, such as light and radio. There are many natural and human-made sources of radiation, the most important of which is the sun. It is often desirable to gather or "concentrate" the radiation emitted by these sources. For example, a solar collector concentrates the sun's light energy to create intense light or heat. Similarly, a television satellite dish concentrates the radio waves emitted by a communications satellite to improve reception.
A common technique for concentrating radiant energy is to focus the energy on a target position with a lens or curved mirror, such as a concave reflector. The problem with this and similar techniques is that the sources of radiant energy are often moving relative to the mirror. For example, as the sun travels across the sky, its position changes relative to a mirror located on the earth. Thus, it has been necessary to provide tracking systems, which move the mirror to track or follow the sun or other moving source of energy.
Typically, these tracking systems use light or heat sensors in combination with motors. The sensors detect in which direction the sun or other energy source is, and the motors position the concentrator. Such tracking systems require elaborate and costly sensing equipment.
Without a tracking system, it is difficult to keep the energy focused on the desired target position. As the energy source moves relative to the concentrator, a significant portion of the radiated energy is lost. There have been attempts to develop passive non-tracking systems. The drawback to these systems, however, is that at any given time, only a small portion of their energy-collecting capability can be used. As a result of this inefficiency, a significant portion of the radiated energy received by the concentrator is lost.